FIG. 10 is a sectional view showing the structure of a photodiode portion (which stores light as charges by photoelectric transfer) and a vertical CCD portion provided in a pixel portion of a CCD that is used as a solid-state image pick-up device for a video camera or the like according to the prior art. In FIG. 10, a first p-type well 2 is formed on an N (100) silicon substrate 1, and a second p-type well 3 and a photodiode n layer 8 are formed in the first p-type well 2. A vertical CCD n.sup.+ layer 4 is formed in the second p-type well 3. An embedded photodiode p.sup.+ layer 9 is formed in the photodiode n layer 8. A p.sup.- area 16 which controls the read of charges from the photodiode n layer 8 to the vertical CCD n.sup.+ layer 4 is formed in one of areas adjacent to the vertical CCD n.sup.+ layer 4 in the second p-type well 3 on the first p-type well 2. A p.sup.+ area 15 which electrically isolates the vertical CCD n.sup.+ layer 4 from the photodiode n layer 8 adjacent thereto is formed in the other area adjacent to the CCD n.sup.+ layer 4. A gate insulating film 6 is formed on the surface of the n-type silicon substrate 1. A polysilicon electrode 7 is formed on the surface of a portion of the gate insulating film 6 which covers the vertical CCD portion 4. The embedded photodiode p.sup.+ layer 9 is formed for the countermeasure against a dark current. The polysilicon electrode 7 functions as an electrode for controlling the read of the charges from the photodiode portion in the vertical CCD portion and the transfer of the read charges, i.e. a read and transfer electrode. The vertical CCD n.sup.+ layer 4 is formed by implantation of arsenic and phosphorus so as to obtain high transfer efficiency.
FIG. 11 is a conceptual view showing the state of impurity diffusion in the vertical CCD n.sup.+ layer (hereinafter referred to as a vertical CCD n.sup.+ area) 4 and the peripheral area thereof in the pixel portion of the solid-state image pick-up device according to the prior art shown in FIG. 10. As shown in FIG. 11, p-type impurities are diffused from a p.sup.- area 16 and a p.sup.+ area 15 to the vertical CCD n.sup.+ area 4 and n-type impurities are diffused from the vertical CCD n.sup.+ area 4 to the p.sup.- area 16 and the p.sup.+ area 15 when forming the p.sup.- area 16 and the p.sup.+ area 15 adjacently to the vertical CCD n.sup.+ area 4 in the solid-state image pick-up device according to the prior art shown in FIG. 10. Accordingly, a true p.sup.+ area 15, an impurity diffusion area 4a in which the p.sup.+ area 15 and the vertical CCD n.sup.+ area 4 overlap each other, a true vertical CCD n.sup.+ area 4, an impurity diffusion area 4b in which the vertical CCD n.sup.+ area 4 and the p.sup.- area 16 overlap each other, and a true p.sup.- area 16 are formed sequentially from the A side corresponding to the line A-B shown in FIG. 11.
FIG. 23 is a sectional view typically showing the structure of a pixel portion of a CCD solid-state image pick-up device according to another prior art. A p-type area 206 is selectively formed on the surface portion of a p-type well 202 in an n-type semiconductor substrate 201 so as to connect an n-type area 209 of a photodiode and an n-type area 204 as a CCD transfer channel. A p-type area 207 is selectively formed on the surface portion of the p-type well 202 so as to connect the n-type area 204 to the n-type area 209 in contact with the n-type area 204 on an end which is not in contact with the p-type area 206. A p-type area 283 is positioned just below the n-type area 204 in the p-type well 202 in such a manner that the n-type area 204 is enclosed by the p-type area 206 for isolation and the p-type area 207 for read drive voltage control. A signal charge non-reading side area is shown by a solid line 286 and a signal charge reading side area is shown by a solid line 285.
According to the method for manufacturing the CCD solid-state image pick-up device according to the prior art described above, boron ions are implanted into the predetermined area of the p-type well 202 by using a photoresist as a mask in order to optimize the read drive voltage, sensitivity, smear, saturation characteristic and the like. Thus, the p-type area 283 is formed. Then, a photoresist is applied, exposed and developed to form a mask. Thus, the n-type area 204 as a CCD transfer channel is formed.
In a CCD solid-state image pick-up device which has widely been used in a video camera, the area for each pixel has been reduced more with the miniaturization of the video camera, the enhancement of precision for a HDTV, and the appearance of a total pixel CCD for multimedia use. For this reason, it is necessary to make the CCD and the photodiode finer without deteriorating D-range, transfer efficiency, sensitivity, smear, after-image and the like. For example, a method for forming an impurity area by using high-acceleration ion implantation and low-temperature heat treatment is very effective to make the CCD and the photodiode finer. By this method, it is possible to prevent unnecessary diffusion of impurities from the n- or p-type area to the undesired area and to form an impurity area having a desired concentration in a portion having a desired depth in the minute area. According to a structure in which the p-type area 283 is formed just below the n-type area 204 as the CCD transfer channel according to the prior art, however, a p-type area having a locally high concentration is formed in the very vicinity of the CCD transfer channel 204 and the photodiode 209. Consequently, various problems arise.
A deterioration in read drive voltage will be described with reference to FIGS. 14 and 15.
FIG. 14 is an enlarged sectional view showing a signal reading side area 285 according to the prior art. An area 233 indicates a p-type area provided just below a CCD transfer channel 204. The p-type area 233 having an end at position 237 is the same as the p-type area 283 according to the prior art shown in FIG. 23. A dashed line 236 indicates a signal reading channel which is formed from a photodiode 209 to the CCD transfer channel 204. Other portions typically show the sectional structure of a pixel of a CCD solid-state image pick-up device according to the prior example shown in FIG. 23.
FIG. 15 shows impurity distribution which is perpendicular to the direction of signal charge transfer and passes through the middle of the CCD transfer channel 204 along the depth direction 284 of an n-type semiconductor substrate 1 according to the prior art. A solid line 241 indicates impurity distribution. The reference numeral 242 denotes a peak concentration of the p-type area 283. The reference numeral 243 denotes a thickness of the p-type area 283.
As shown in one-dimensional impurity distribution 241, the p-type area 283 has local distribution at a high concentration. For example, if the dose of boron is 2.0.times.10.sup.12 cm.sup.-2, the peak concentration 242 is about 3.8.times.10.sup.16 cm.sup.-3 and the thickness 243 is about 0.7 .mu.m. For this reason, a depleted layer is hindered from expanding from the CCD transfer channel 204 with an increase in voltage of the gate electrode 208 when reading signals. Consequently, it is hard to form a signal reading channel 236 from the photodiode 209 to the CCD transfer channel 204. Thus, the read drive voltage is raises to generate after-image.
There is a problem in that the smear characteristic is considerably deteriorated on a reading side 285. According to the method of the prior art, the CCD transfer channel 204 and the p-type area 283 are formed by separate masks. Consequently, a variation in mask alignment is inevitably caused. For this reason, the read drive voltage, smear and saturation characteristic are fluctuated due to the variation in mask alignment.